The present invention relates to print a hammer driving means in impact printers.
In the impact printer used in a desk-top electronic calculator or an electronic cash register, the print hammer is snapped into engagement with a type character on a type drum or on a type belt to impress the type upon a recording paper disposed between the type character and an inked ribbon. In a known impact printer described in FIGS. 1 and 2 of U.S. Pat. No. 3,848,527, a hammer spring is provided for driving a corresponding print hammer and a timing member such as a rotary timing cam is provided for tensioning or winding up the spring and for releasing the print hammer.
The spring is always held in the tension state and the print hammer is released at a predetermined time, so that the hammer may be driven by the energy stored in the spring to strike the type character for printing. The spring is wound up immediately after the printing and the hammer is held in the waiting position. In the waiting position, all print hammers in the machine are forced into frictional engagement with the timing cam by the tensioned springs. Accordingly, the timing cam rotates with slipping on the hammers during operation, which entails a considerable energy loss on the power source.
Further, the frictional engagement between the timing cam and the hammer produces a harsh noise during the waiting period. It should be noted that the waiting period occupies a considerable part of one printing cycle.
In another known impact printer shown in FIG. 3 of U.S. Pat. No. 3,848,527, print hammer driving means are so arranged that power of the motor is directly transmitted to the print hammer. The impact printer is provided with a motor-driven snatch roll and print hammers are provided adjacent the snatch roll. Each hammer is caused to be engaged with the snatch roll at a predetermined time, so that the hammer may be moved to the type drum for performing the impact printing. Since the snatch roll rotates without engagement with the print hammer in the waiting period, problems of energy loss and noise are solved. However, impact force for printing varies with variation of rotational speed of the motor due to variation of voltage of power source and to variation of load in the transmission system. As a result, defective print such a irregular printing is caused. Therefore, it is necessary to use a special motor with a constant rotational speed or to provide a governor for compensating the speed variation. Consequently, the device would be increased in size and would be expensive.
The above described disadvantages are noticeable in a battery powered impact printer of a small size and low power consumption. More particularly, in the impact printer of the hammer spring deflexion type as described above, friction between the hammer and the timing cam consumes a considerable amount of electric power in the waiting period, which causes a decrease in the life time of the battery. In the snatch roll type printer, it is necessary to use a motor having a constant rotational speed. However, a battery powered motor having such a characteristics cannot be obtained at the present time. In addition, when the snatch roll engages the print hammer, a great energy is required for driving the hammer. However, it is difficult to supply such a momentary great energy to the snatch roll by means of a battery powered motor.